The present invention relates to an electric power steering apparatus used for lessening an operational force (steering force) of a steering wheel, and more particularly to an improvement in an electric power steering apparatus wherein an electric motor in used as means for producing a steering assistance effort.
Steering apparatus generally called "power steering arrangement" have been widely embarked on various kinds of vehicles etc. including compact cars in recent years. Such power steering apparatus lessen a steering force of a driver to enable a simple and nimble steering operation and can exhibit their effect in lessening fatigue etc. of the driver. In general, oil pressure type power steering apparatus have been used in the art as apparatus of this kind. Recently, various electric type power steering apparatus utilizing an electric motor etc. without making use of oil pressure have been proposed.
The drawbacks with the first-mentioned oil pressure type power steering apparatus are as follows. They require not only an oil pressure cylinder for generating a steering assistance but also an oil pressure pump, a flow path switch valve, and oil pressure piping system etc. connecting these parts to each other, resulting in a large number of parts. Further, the employment of such apparatus results in not only complicated structure in each part and difficulty and troublesomeness in manufacturing and assembling work, but also large-sized apparatus in the entirely and increased weight.
To overcome these problems, there have been recently employed various kinds of electric power steering apparatus in which an electric motor which can be used by connecting it to a battery embarked on a vehicle and a controller with a simple electric wiring is used as means for generating a steering assistance. Such electric power steering arrangement make it possible to simplify the configuration of each part of the apparatus and to make the entirely of the apparatus small and compact.
However, in order to realize a power steering apparatus using the above-mentioned electric motor as means for generating a steering assistance, there arise various problems in respect of motor control and response etc. Accordingly, such electric power steering apparatus have been beyond realization for the present time.
It is essential for realization of the above-mentioned electric power steering apparatus to simply and properly detect various kinds of running conditions e.g. a steering force or a steering angle at maneuvering conforming to a steering operation of a driver and a running speed etc. of a vehicle to suitably control and drive an electric motor in response to the value detected, thus providing a necessary steering assistance.
Particularly, vehicles on which a power steering apparatus of this kind is embarked are required so that they can output a large steering assistance in order to produce an extremely light steering operational force in the case of carrying out a steering operation when a vehicle is stopped, i.e., steering with the wheel being laid on ground or a steering operation when a vehicle runs at a low speed. When the electric power steering apparatus are used in such a case, they can exhibit their effect. However, on the other hand, where a vehicle runs at a high speed, if a large steering assistance effort is caused to be produced at the time when a vehicle runs at a low speed as previously mentioned or the like, an operating force of a steering wheel is too light, resulting in the problem that uneasiness of a driver is urged. This is not desirable in an attainment of an appropriate driving sense from a practical point of view. Conversely, it is required to provide a structure which allows steering assistance to be reduced to apply a relatively large weight to the steering wheel, i.e., to increase a steering force.
To meet such a requirement, it is required to drive the above-mentioned various kinds of power steering apparatus with they being controlled depending upon running conditions and running speed of a vehicle, magnitude of a steering force and magnitude of a steering at maneuvering. To realize this, various kinds of detector devices must be additionally provided.
Further, brawbacks with electric power steering apparatus of this kind are as follows. When an electric motor is employed, it is difficult to obtain operating characteristics conforming to a steering operation of a steering wheel due to inertia etc. of the motor. In addition, where there is a need to return a steering wheel steered to the original position, because of provision of a reduction gear mechanism between a steering shaft and the motor, it is required to detect a steering condition of a driver who attends to such an operation to rotate the motor in a reverse direction. Namely, where a steering road wheel is returned from swivelling condition to straight advancing condition in steering apparatus additionally provided with a power steering device, self-aligning torque (force applied so that a vehicle advances in a beeline when the vehicle being running) produced due to reaction force when the steering load wheel is in contact with ground is utilized. However, with the above-mentioned motor, friction is produced in its brush or the like, thus making the reverse rotation of a rotor insufficient. Accordingly, there arises a need of compensating for this insufficiency.
Htherto, detector devices of various design have been known as the above-mentioned detection mechanism. However, any one of them has drawbacks in structure or operating performance. This is serious e.g. in connection with a steering force detection mechanism to detect a steering request of a driver in respect to the steering wheel. Particularly, there arise various problems in respect of the detection method, the arrangement position of the structure and the detection performance etc.
For instance, a power steering apparatus additionally provided with a steering force detection mechanism of on/off type below the column shaft has been proposed. However, such a configuration allows the lower end of the shaft which is not enough from a view point of space for arrangement to be large, giving rise to complicated structure and troublesomeness in assembly work. In addition, because detector means of the contact type is employed, when repeatedly used, complicated adjustment work is required and a sufficient durability cannot be obtained. There has been also known another steering force detector mechanism provided with switching means of a contact strip at a portion joining the shaft of the steering wheel. However, even where such a detector mechanism is employed, because the switch is of the contact type, when repeatedly used, there arise problems in regard to the durability and the reliability in operation. This is serious when the last-mentioned detector mechanism is practically used. Further, torque sensors well known in the art have employed a strain gauge in most cases. However, such a detector means has difficulty in the reliability in respect to environment changes of e.g. temperature or humidity etc. and the reliability in respect to use over a long time. Therefore, such strain gauge type sensors are not practically acceptable. There are many problems when conventional mechanisms are practically used.
Accordingly, when embodying the electric power steering apparatus of this kind, it is required to properly detect the above-mentioned steering torque with a simple detector means to securely control a motor. Further, one problem occuring when embodying the above-mentioned electric power steering apparatus resides in the layout e.g. how the electric motor and the drive force transmission mechanism therefor are joined to the ordinary steering shaft. Namely, it is required to satisfy the following points. First is that the structure of the entirely of the apparatus is simple and manufacturing or assembling work etc. is easily carried out. Second is that the entirety of the apparatus is small and compact. Third is to adaptably incorporate the electric power steering apparatus into a vehicle.
Since the power steering apparatus of this kind is recently employed particularly in compact cars etc. limited in space, it is important to meet the abovementioned requirement.
However, on the other hand, the electric power steering apparatus which have been known in the art are all complicated in structure, failing to realize that the entirety of the apparatus is small and compact. Particularly, there arise problems when the power steering apparatus of this kind is actually incorporated into a vehicle.
For instance, an apparatus as described in the Japanese patent publicaion Tokkaisho No. 59-63264 etc. has been proposed wherein the apparatus is condigured to transmit a drive force from an electric motor to a portion of a steering shaft between a steering road wheel through a reduction gear mechanism. However, such a structure requires the provision of a body for the motor and the transmission system in additon to the ordinary steering gear body, with the result that the entirety of the apparatus becomes large, thus failing to satisfy the above requirement.
Further, an apparatus as shown in the Japanese patent publication Tokkaisho No. 57-134361 etc. has been also proposed wherein the above-mentioned electric motor and the transmission mechanism therefor are additionally provided in series at the side portion of the steering gear body to transmit a steering assistant force to an output shaft on the side of a steering road wheel joined to an input shaft on the side of a steering wheel through a torsion bar. Furthermore, an apparatus as shown in the Japanese patent publicaton Tokkaisho No. 57-47251 etc. has been proposed when he electric motor is directly joined to the extented end of the output shaft. However, these apparatus mentioned above cannot avoid that the entirety of the apparatus becomes large, and have the drawback that the structure becomes complicated. Therefore, it is strongly required to take any countermeasure.